Pneumatic/hydrostatic riser tension

Abstract

The present invention provides heave compensation through a riser tensioner for a marine riser ( 8 ) supported from a sea vessel ( 100 ) supporting a cylinder barrel ( 11 ) with a mainly vertically arranged cylinder axis ( 110 ). The cylinder is provided with a piston ( 12 ) with an outer diameter corresponding to the cylinder barrel ( 11 ) and arranged coaxially inside the cylinder barrel ( 11 ). The cylinder barrel ( 11 ) has a larger diameter than the riser ( 8 ). The cylinder barrel ( 11 ) has an upper end ( 5 ) communicating with the atmosphere through a vent channel ( 6 c ) from a volume ( 6 a ) above the piston ( 12 ), the vent channel ( 6 c ) and the volume ( 6 a ) arranged for being air-filled, excerting an atmospheric pressure downwards on the piston ( 12 ). The cylinder barrel ( 11 ) has a lower end ( 112 ) arranged for being submerged below the sea surface and being open towards the sea The piston ( 12 ) is subject to a hydrostatic pressure from below from the sea, excerting a net upwards force supporting and balancing the weight of the riser ( 8 ).

1 . Tensioner ( 0 ) for a marine riser ( 8 ) supported from a vessel ( 100 ) floating at the surface ( 6   g ) of a sea ( 6   f ), comprising 
 a cylinder barrel ( 11 ) having an upper end ( 5 ) and a lower end ( 112 ), supported by said vessel ( 100 ) and having a cylinder axis ( 110 ) mainly for vertical arrangement, and having a piston ( 12 ) with a diameter being larger than a diameter of said riser ( 8 );    said piston ( 12 ) separating a variable volume ( 6   a ) filled with air ( 6   d ) communicating with the free atmosphere ( 6   e ) through a vent channel ( 6   c ) above said piston ( 12 ), from a water-filled variable volume ( 6   b ) below said piston ( 12 ), said lower end ( 112 ) of said barrel ( 11 ) arranged for being submerged below the sea surface and being open towards the sea ( 6   f ), with said piston ( 12 ) being submerged to a depth essentially below said sea surface ( 6   g );    said air ( 6   d ) of said volume ( 6   a ) excerting pressure downwards on said piston ( 12 ), said piston ( 12 ) being subject to a hydrostatic pressure from below from the sea ( 6   f ), excerting a net upwards force for supporting and balancing the weight of said riser ( 8 ).    
     
     
         2 . Tensioner according to  claim 1 , said upper end ( 5 ) at the barrel ( 11 ) having a cylinder top ( 50 ) with a riser seal ( 6 ), said vent channel ( 6   c ) having a lower channel end ( 61 ) in said volume ( 6   a ) and an upper channel end ( 62 ) in said atmosphere ( 6   e );  
     
     
         3 . Tensioner according to  claim 1 , said piston ( 12 ) having an axial opening ( 13 ) with a diameter corresponding to said diameter of said riser ( 8 ), with a rim shoulder ( 131 ) of said axial opening ( 13 ) arranged for supporting a riser hangoff ring ( 3 ) on said riser ( 8 )  
     
     
         4 . Tensioner according to  claim 2 , said riser seal ( 6 ) having diameter allowing passage of an outer casing ( 7 ) around said riser ( 8 ), said outer casing ( 7 ) extending from said piston ( 12 ) to above said sea surface ( 6   g ), said outer casing ( 7 ) comprising said vent channel ( 6 ).  
     
     
         5 . Tensioner according to  claim 1 , a periphery of said piston ( 12 ) being provided with one or more piston rings ( 9 ) against the inner wall of said barrel ( 11 ).  
     
     
         6 . Tensioner according to  claim 1 , said piston being provided with safety valves ( 10 ) arranged in the lower part of said outer casing ( 7 ) for releasing parts of said air ( 6   f ) from said air volume ( 6   a ) should the pressure in said air volume ( 6   a ) exceed a given pressure limit (Pr) during a deep surge of heave movement.  
     
     
         7 . Tensioner according to  claim 4 , with a drain sump ( 21 ) in the top surface of said piston ( 12 ), said drain sump ( 21 ) feeding a water lift pump ( 2 ) connected to a water drain line ( 20 ) extending up through said outer casing ( 7 ) for draining undesired water from the bottom of said air-filled volume ( 6   a ).  
     
     
         8 . Tensioner according to  claim 2 , with a spare piston ( 12 ′) arranged immediately below said piston ( 12 ), inside said barrel ( 11 ).  
     
     
         9 . Tensioner according to  claim 7 , with one or more spare lift pumps ( 2 ) arranged connected to said drain sump ( 21 ) and to said water drain line ( 20 ).